1. Field of the Invention
The present invention relates to a gripping device and more particularly to a prestress-adjustable piezoelectric gripping device.
2. Description of the Related Art
Micro-Grippers play an essential role in micro assembling systems, but a micro actuation device is an even more important technical module; it is indispensable if a strong gripping effect is to be achieved. Generally, the micro actuation devices or precision actuation devices are simply achieved by utilizing the micro deformation characteristic of a piezoelectric element. Common piezoelectric elements often adopt actuation modes of a longitudinal effect and a transverse effect (that is, horizontal or perpendicular configuration, and from the perspective of deformation, both longitudinal and transverse variations actually exist simultaneously, although the longitudinal and transverse effects are respectively adopted as the actuation modes) depending on different material polarization directions (P) and applied electric field directions (E). However, conventional micro grippers using a MEMS process have the following problems: an insufficient gripping velocity, an inadequate gripping force, and a short driving displacement.
Referring to FIG. 1, a conventional piezoelectric element is shown with a longitudinal effect actuation; the piezoelectric element 1 has a polarization direction (P) and an applied electric field direction (E), both of which are in a lengthwise direction of the conventional piezoelectric element 1 with a longitudinal effect actuation (i.e., configured in parallel). Under the stress generated in the polarization direction (P) and the electric field direction (E), the conventional piezoelectric element 1 with a longitudinal effect actuation is deformed longitudinally (as shown by the dashed line), so as to produce a longitudinal actuation force.
Referring to FIG. 2, a conventional piezoelectric element 2 is shown with a transverse effect actuation; the piezoelectric element 2 has a polarization direction (P) opposite an applied electric field direction (E). Both the polarization direction (P) and the applied electric field direction (E) are in a widthwise direction of the conventional piezoelectric element 2 with transverse effect actuation (i.e., configured in parallel). Under the stress generated in the polarization direction (P) and the applied electric field direction (E), the conventional piezoelectric element 2 with a transverse effect actuation is transversely deformed (as shown by the dashed line), so as to produce a transverse actuation force.
The above conventional piezoelectric element 1 with a longitudinal effect actuation and the conventional piezoelectric element 2 with a transverse effect actuation generate longitudinal effects and transverse effects respectively, which are characteristics generated when an external electric field direction (E) and a polarization direction (P) are configured in parallel. Unfortunately, the above two driving configurations cannot produce a shear effect while generating longitudinal and transverse effects. Therefore, with a single piezoelectric element, although a high actuation precision or a micro actuation effect can be achieved, the realized driving displacement is only up to tens of micrometers (μm). What's worse, some piezoelectric elements can only actuate up to the level of sub micrometers (sub-μm), and thus, it is rather difficult to grip a micro element with a larger size (e.g., more than 100 μm).
In addition, in order to prolong the driving displacement of the above conventional piezoelectric element 1 with a longitudinal effect actuation and the conventional piezoelectric element 2 with a transverse effect actuation, a plurality of piezoelectric elements must be stacked together.
FIGS. 3A and 3B show a conventional stacked piezoelectric actuation device 3, which includes the conventional piezoelectric element 1 with a longitudinal effect actuation and the conventional piezoelectric element 2 with a transverse effect actuation. The conventional piezoelectric element 2 with a transverse effect actuation is disposed on the conventional piezoelectric element 1 with a longitudinal effect actuation. FIG. 4A shows a first driving signal input to the conventional piezoelectric element 1 with a longitudinal effect actuation, and FIG. 4B shows a second driving signal input to the conventional piezoelectric element 2 with a transverse effect actuation. The first driving signal and the second driving signal have a phase difference.
As shown in FIGS. 3A to 4B, upon receiving the first driving signal, the conventional piezoelectric element 1 with a longitudinal effect actuation generates a longitudinal effect for longitudinal motions. Upon receiving the second driving signal, the conventional piezoelectric element 2 with a transverse effect actuation generates a transverse effect for transverse motions. The respective dashed lines in FIG. 3B indicate the conventional piezoelectric element 1 with a longitudinal effect actuation and the conventional piezoelectric element 2 with a transverse effect actuation before deformation. By controlling the first and second driving signals with a phase difference to respectively drive the conventional piezoelectric element 1 with a longitudinal effect actuation and the conventional piezoelectric element 2 with a transverse effect actuation, the conventional stacked piezoelectric actuation device 3 generates an approximately rectangular-shaped or ellipse-shaped movement track to achieve the effect of pushing or actuation.
The actuation mode of the above conventional stacked piezoelectric actuation device 3 is the most common and convenient aspect practiced among micro piezoelectric actuators. However, in order to generate a desirable movement track, two piezoelectric materials are required and the electric signals with two phases (the first and second driving signals) must be accurately matched, which is rather complicated in terms of hardware implementation.
Therefore, there is a need to provide a prestress-adjustable piezoelectric gripping device to solve the above problems.